1. Field of the Invention
The present invention relates to a light quantity adjusting device suitable for use in an image taking apparatus such as a video camera or a digital still camera, and to a technique capable of suppressing a deterioration of optical performance even in an image pickup element having a small pixel pitch.
2. Related Background Art
For an image taking optical system of an image taking apparatus such as a video camera, a light quantity adjusting device for changing an aperture diameter produced by a plurality of diaphragm blades to adjust a quantity of light is used. In such a diaphragm apparatus, when the aperture diameter becomes too small, the deterioration of optical performance due to light diffraction becomes a problem.
Therefore, in order to prevent the aperture diameter from becoming too small even in the case of an object receiving sufficient light, a light quantity adjusting device using diaphragm blades in combination with an ND (neutral density) filter has been proposed and used in practice.
According to Japanese Patent Publication No. 2592949, a diaphragm apparatus is disclosed in which an ND filter is bonded to diaphragm blades so as to be positioned within an aperture formed by the diaphragm blades and have a plurality of regions for which uniform transmittance is set, and the transmittance is set so as to gradually increase from the outside of the aperture to the inside thereof.
According to Japanese Patent Application Laid-Open No. 52-117127, a diaphragm apparatus is disclosed in which mechanical diaphragm blades are moved during a control from a full open state to a state with a predetermined aperture area, and in small aperture control where the aperture is not larger than a predetermined diaphragm value, an ND filter whose transmittance is continuously changed according to the density is entered into the aperture successively from a filter portion having high transmittance.
In Japanese Patent Application Laid-Open No. 2000-106649, the influence of transmittance of an ND filter having a plurality of density regions on an optical performance is described and an image pickup apparatus having an exposure control mechanism for which measures are taken against the influence is disclosed.
In the cases of these conventional proposals, with respect to the main factor for the deterioration of optical performance in a middle aperture state between a full open aperture state and a small aperture state, it is considered that the influence of diffraction due to a difference of the transmittance of the ND filter covering the aperture portion produced by the diaphragm blades is dominant. Thus, in the case of the ND filter having a plurality of density regions, measures against the influence of diffraction have been proposed with attention to transmittances and areas of the respective regions of the ND filter.
On the other hand, with respect to a factor for the deterioration of optical performance in the middle aperture state, there is not only the influence of diffraction due to a difference of the transmittance of the ND filter but also the influence of a phase difference of transmission wave front resulting from a thickness component of the ND filter.
It has been experimentally known that an optical performance is deteriorated when a part of a diaphragm aperture portion is covered with a filter having a thickness. However, there is known no example in which it is examined as to how the optical performance is influenced by a filter thickness component and specific countermeasures are taken therefor.
According to Japanese Patent Application Laid-Open No. 06-265971, a structure in which an ND filter having a transparent portion and a portion in which transmittance is changed continuously or stepwise is moved in a state in which it covers the entire fixed circular diaphragm aperture, thereby adjusting a quantity of light transmitted, has been proposed as a measure for avoiding the influence of the thickness of the ND filter on the optical performance.
However, the invention according to Japanese Patent Application Laid-Open No. 06-265971 focuses attention on only a large phase difference between a portion in which light passes through a filter member and a portion in which light does not pass through the filter member. Thus, there is no suggestion as to the problem of, and measures against, a minute phase difference of transmission wave front up to about two times larger than a wavelength of light λ, which results from a change in minute thickness or a change in minute refractive index which will occur in order to provide a change in transmittance when actually realizing an ND filter in which transmittance is changed. According to a study by the present inventor(s), it was found that a minute phase difference of transmission wave front of no more than the order of wavelength of light has a very large influence on the optical performance under certain conditions.
Also, it was found that there is a difference between how the influence of the phase difference of transmission wave front on the optical performance appears and how the influence of a density difference between adjacent regions of the ND filter with different transmittances on the optical performance appears, thus greatly affecting the optical performance under certain conditions by the synergism of the two components, namely the phase difference of transmission wave front and the density difference.